The IEEE 802.11 standard outlines the media access control (MAC) and the Physical Layer (PHY) layer specifications for wireless LAN. Two major variants have emerged from the IEEE 802.11 specifications, namely the 802.11b and 802.11a standards. More specifically, the IEEE 802.11b emerged from 802.11 as the “high rate” and Wi-Fi™ standard specifying the DSSS system to operate at 1, 2, 5.5 and 11 Mbps. The IEEE 802.11b compliant devices operate in only the 2.4000 GHz ISM band between 2.4000 and 2.4835 GHz, while IEEE 802.11a, hereinafter referred to as ‘802.11a’, compliant devices operate at the higher 54 Mbs rate in the 5 GHz band, which supports even higher data rates owing to the implementation of Orthogonal Frequency Division Modulation (OFDM). 802.11a specifies the PHY layer operating in the 5 GHz band using the orthogonal frequency division modulation (OFDM) scheme to modulate the data. The orthogonal frequency division multiplexing scheme allows high data rate transmission by dividing the data streams into a number of lower data rate streams and then transmitting each one on a separate sub channel. An OFDM signal consists of a sum of subcarriers that are modulated by using Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). The signals are generated by Fourier Transformations.
In order to demodulate the received signal, an 802.11a receiver has to synchronise. An OFDM synchronisation method performs two main tasks. First, the method must derive an optimum sampling instance and hence determine the symbol boundaries to minimise the effects of inter-subcarrier interference (ICI) and intersymbol interference (ISI). Secondly, the method estimates and corrects the carrier frequency offset of the received signal to avoid ICI.
Owing to low receiver sensitivities there is a loss in the peak magnitude, and multipath leading to distortion. The current invention provides for a solution to the optimum sampling instance for determining the symbol boundaries to minimise the effects of inter-subcarrier interference (ICI) and intersymbol interference (ISI). This relates to one aspect of the synchronisation method.
A typical example of OFDM physical layer is the IEEE 802.11a standard, hereinafter referred to as ‘802.11a’. The PLCP (Physical Layer Convergence Procedure) preamble consists of pre-defined sets of OFDM signals, which are intended for use in synchronisation. In accordance with the 802.11 medium access control (MAC) protocol, the receiver constantly scans the medium to establish whether or not it is busy or free for transmission. The receiver uses the preamble as a reference signature to establish the presence of a packet on air. Once the preamble is recognised at the receiver, the synchronisation method establishes an appropriate sampling interval and the rest of the packet is demodulated and decoded by the modem PHY.
The skilled addressee would acknowledge that a sequence of 10 identical symbols as shown in FIG. 1, each constituting a pre-defined short training sequence (STS), is transmitted as the preamble of a frame. Typically, the receiver identifies the reception of a packet by cross correlating the received signal block against the STS. The OFDM signal (t1, t2, . . . ) is generated using the standard equations that would be familiar to a person skilled in the art. The correlation process produces peaks for every identical short training sequence received. By implementing a threshold detector, it is possible to identify the position of the periodic correlation peaks, and hence the detection of an OFDM frame and the consequent symbol boundaries.
However, in wireless communications, the transmit signal is distorted due to the composite of multipath fading and signal shadowing. In addition, the auto-correlation profile of the STS manifests notable pre and post cursors either side of the correlation peaks, which obscures the correlation peaks required for synchronisation. In multipath conditions, the cross correlation of the received signal preamble with the STS shows dominant peaks before the desired correlation peak. Consequently, a threshold based peak detector fails to isolate the correct peak and hence the symbol boundaries are incorrectly identified. Furthermore, frame start position is also incorrectly calculated.
The current invention addresses the problem of synchronisation based on short training sequences for the IEEE 802.11a standard; however, the ensuing solution is equally applicable to other OFDM schemes. Hence, to overcome the limitations of the methods for synchronisation of receivers known to the skilled addressee, the current invention provides for a solution to the problems of synchronisation within any packet switched network or system using short training sequences, wherein the PHY layer packets are transmitted over air are synchronised at the receiver by exploiting the presence of a predefined packet preamble.